Logic of retrieving netdev speed from net_device and translating it to
IB speed is implemented in rxe, in usnic and in bnxt drivers.

Define new function which merges all.
Signed-off-by: NYuval Shaia <yuval.shaia@oracle.com>
Reviewed-by: NChristian Benvenuti <benve@cisco.com>
Reviewed-by: NSelvin Xavier <selvin.xavier@broadcom.com>
Reviewed-by: NMoni Shoua <monis@mellanox.com>
Signed-off-by: NDoug Ledford <dledford@redhat.com>